Heretofore, various types of liquid-gas contact apparatuses have been employed to remove sulfur dioxide from exhaust smoke using a wet-gas method. These would typically be used to remove harmful substances like sulfur dioxide from the exhaust smoke of a coal-burning boiler. One such apparatus, a previous design by the present petitioners which employs a liquid-column method, is described in Japanese Utility Patent Publication (Koukai) 59-53828.
This apparatus has a number of spray nozzles arranged in an absorption tower. An absorption liquid such as lime slurry is sprayed upward from these spray nozzles to form an absorption column. When exhaust smoke is forced into the center of this flow, the sulfur dioxide in the smoke is absorbed and particulate like fly ash are effectively removed.
A basic design for such a tower is shown in FIG. 24 (A). In the upper portion of absorption tower 2 is exhaust path 8; in its lower portion is smoke inlet 3, the entry port for exhaust gases 1. A number of rows of head pipes 190 are arranged in the lower portion of absorption tower 2. On the pipes 190 are numerous upward-facing spray nozzles, which may, for example, be arranged in a matrix, as shown in FIG. 24(B).
The bottom of absorption tower 2 is made into a funnel shape to form liquid recovery vessel 56. Here the lime slurry or other absorption liquid 5 is collected, after which it is routed to liquid storage tank 57 by pump 21a. This collected absorption liquid 5 is again circulated through spray pump 21b, volume control valve 60 and head pipes 190 back to spray nozzles 4.
The spray nozzle array consisting of all the upward-facing spray nozzles 4 arranged in the matrix forces absorption liquid 5 upward and causes it to assume the form of liquid column jets 5a. At the same time, exhaust gases 1 are brought in via the smoke inlet 3 and forced upward. The flow carries these gases along with the jets of absorption liquid 5 to the top of the tower, where they must pass through jets 5a, now distributed in an umbrella shape. In this way the liquid and vapor are brought into contact with each other.
Subsequently, mist eliminator 6, located in the top of absorption tower 2 around the highest point reached by the jets, separates the absorption liquid 5 which has accompanied exhaust gases 1 and recycles it into liquid storage tank 57. The liquid 5 which falls directly into recovery vessel 56 is transported by recirculation pump 21a into liquid storage tank 57.
With a liquid-vapor contact apparatus of this configuration, when pump 21b is operated, absorption liquid 5 travels through volume control valve 60 and head pipes 190 and is sprayed upward through spray nozzles 4. The exhaust gases 1 which are introduced through entry port 3 are forced to pass through jets 5a to effect liquid-vapor contact. The processed (scrubbed) exhaust gas 7, from which the sulfur dioxide and other noxious components have been removed, is expelled via exhaust path 8.
When this technique is used, by which absorption liquid 5 is sprayed upward, the vapor and liquid are in contact for the entire time that liquid 5 travels up and down the tower. In addition, when liquid 5 reaches the top and spreads into an umbrella shape for its descent, it assumes the form of droplets. This enhances the effect of liquid-vapor contact. When the exhaust gases contain only minimal sulfur dioxide, greater operating economy can be achieved by changing the height of the column of liquid. This method offers a number of benefits over what is known as the packing method, in which the liquid flows into a tower packed with a grid and is there brought into contact with the gases. One such benefit is that with the jet-spray method, the channel for the liquid is unlikely to become saturated.
Also, when the jet-spray method is used, operating spray pump 21b will cause the absorption liquid 5 collected in recovery vessel 56 or tank 57 to be recirculated to head pipes 190, and the pressure of the spray can be adjusted so that the liquid 5 attains a specified height from spray nozzles 4.
For the sake of simplicity, spray pump 21b in the drawing is represented as a single entity. However, in a real situation a number of pumps would be used, posing a problem in terms of compactness as well as cost of equipment and operation.
Furthermore, in order to improve the efficiency of liquid-vapor contact between the exhaust gases and the absorption liquid, a large number of spray nozzles are needed to break the water into minute particles. This is the purpose of the array shown in FIG. 24(B), in which many spray nozzles are arranged in the form of a matrix. We see, then, that this design is costly in terms of the equipment it requires.
It was to address this problem that the designs disclosed in German patent DE-A-1769945 and Japanese patent publication (Kohyo) 9-507792 were proposed. In these apparatuses, a liquid storage tank is provided for the slurry supplied and recirculated to the spray nozzles. The level of the liquid inside the tank is kept higher than the level of the spray nozzles. The absorption liquid sprayed from the spray nozzles is made to accompany the gases to the top of the absorption tower, where the liquid and gases are separated. The separated liquid is kept in the tank, and the gravity differential between the surface of the liquid in the tank and the spray nozzles is used to spray the slurry from the spray nozzles. Without the use of a spray pump, then, employing only the gravity differential, the slurry can be sprayed from the spray nozzles and recirculated.
However, with these prior art designs, the surface of the liquid in the liquid storage tank must be higher than the level from which the spray nozzles spray the liquid inside the absorption tower. Generally, the absorption tower is relatively high, so the tank must be placed even higher in order to have the surface of the liquid above the height of the spray nozzles.
Also, with gas cleaners such a desulfurization apparatus, it is normal for the load on the boiler or other source of the gases to vary. In both these apparatuses, when the gas flow decreases, the drop in the flow velocity will result in less fluid being entrained. This makes it impossible to achieve smooth and consistent contact between the liquid and the gas flow. The sulfur dioxide and particulate will not be effectively removed from the smoke, and the absorption liquid will not reach the top of the tower. This will make it very difficult to return the absorption liquid to its tank, and the level in the tank will gradually decrease until finally it may happen that the liquid can no longer circulate under the force of its own weight.
With both these apparatuses, the velocity at which the liquid is sprayed, or to put it another way the height to which it is sprayed, is directly proportional to the velocity of flow of the exhaust gases. In FIG. 13, the vertical axis represents the exhaust gas velocity, and the horizontal axis represents the operation time. As can be seen in FIG. 13, when the combustion capacity of a combustor is small, as from time T.sub.1, the start-up time of the absorption tower, to time T.sub.2, its shut-down time, the flow velocity of the exhaust gases will decrease in that time, and it will prove impossible to raise the liquid sprayed from the spray nozzles above a standard level. The horizontal broken line in FIG. 13 represents the minimum loading velocity. As a result, the entire volume of the spray does not reach the mist eliminator in the top of the tower. Instead it falls into the recovery vessel and accumulates there excessively.
Thus it was that in order to achieve continuous recirculation of the absorption liquid in such prior art apparatuses, recovery vessel 56 had to be of a substantial size, and recirculation pump 21a, which would be used only during start-up and shut-down of absorption tower 2, had to be a large-capacity pump. These requirements were linked to a needless increase in the cost of the equipment.
In both of these apparatuses, the mist (i.e., water droplets) which entrains with the exhaust gases and so absorbs the target components such as sulfur dioxide strikes the folded panel of the mist eliminator in the top of the tower and then drips down. In this way the absorption liquid is continuously recycled. In prior art apparatuses such as those we have been discussing, the flow velocity of the gases in the tower is normally around 4 to 5 m/s, a speed which allows the mist eliminator to capture the water. Recently, however, there has come to be a growing demand for a flow velocity of over 5.5 m/s, which would improve the processing capacity and reduce the space requirement.
At a velocity over 5.5 m/s, not all the water droplets which reached the top of the tower would be captured by the mist eliminator. The droplets which were still free would be exhausted to the exterior along with their entrained gases. This would be undesirable regardless of whether the gases were being released into the atmosphere or sent to a final stage processing apparatus.
Further, at a velocity over 5.5 m/s, the volume of absorption liquid entrained to the gases which reaches the mist eliminator greatly increases. The liquid which should drip down from the mist eliminator instead forms a vortex at its inlet and remains there (i.e., a zone is created in which the scattered absorption liquid collects). This severely compromises the mist elimination function. As less mist is captured, the water droplets in the collection zone entrain with the exhaust gases and disperse once again. The volume of mist escaping through the smoke flue increases.